Screen provided with retroreflective microstructures

ABSTRACT

A retroreflective screen including a first film having a surface including a plurality of microrecesses, each microrecess having a bottom substantially parallel to the mean plane of the screen and first and second lateral walls substantially orthogonal to each other and substantially orthogonal to the bottom, the first and second lateral walls and the bottom of the microrecess joining at a same point and forming a trihedron.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.15/239,956, filed Aug. 18, 2016, which claims priority to French patentapplication number 15/58036, each of these applications is incorporatedherein by reference in its entirety to the maximum extent allowable bylaw.

BACKGROUND

The present disclosure relates to the field of image display systems ontransparent surfaces such as windshields of vehicles, particularly ofmotor vehicles. It more particularly aims at a screen provided withreflective microstructures adapted to such a system, at a method offorming such a screen, and at a mold for forming such a screen.

DISCUSSION OF THE RELATED ART

The applicant has recently provided in French patent application NoFR14/53404 filed on Apr. 16, 2014 as well as in the correspondinginternational patent application No PCT/FR2015/050956 filed on Apr. 9,2015, a system of image display on a windshield using a partiallytransparent and partially retroreflective screen coating the innersurface of the windshield.

FIG. 1 is a simplified cross-section view of such a system. The systemcomprises a screen 103 coating the inner surface of a windshield 101,that is, its surface facing the inside of the vehicle, and a projector105 capable of being mounted on the head of a user 107, for example, thevehicle driver. Projector 105 is capable of projecting an image on allor part of the surface of screen 103 facing the inside of the vehicle(that is, opposite to windshield 101). Screen 103 is partiallytransparent and partially retroreflective. More particularly, screen 103is capable of retroreflecting that is, of reflecting towards its sourcelight projected on its surface facing the inside of the vehicle, and ofgiving way with no significant alteration to light originating fromwindshield 101, that is, from the outside of the vehicle. Screen 103thus has a transparency function, enabling the user to see the outerscene through windshield 101 from the inside of the vehicle, and aretroreflection function, enabling the user—whose pupils are close toprojector 105—to see, overlaid on the outer scene, an image generated byprojector 105.

FIG. 2 is a cross-section view showing in further detail screen 103 ofthe system of FIG. 1.

Screen 103 is formed of a film of a transparent material having anapproximately smooth surface 201 a and having a surface 201 b oppositeto surface 201 a exhibiting substantially identical protrusions 203regularly distributed across the film surface. Each protrusion 203 hassubstantially the shape of a cube corner, that is, of a trihedroncomprising three mutually perpendicular triangular lateral surfacesjoining at a same point, or summit, and opposite to the summit, a base,for example in the shape of an equilateral triangle. Each protrusion 203has its summit pointing towards the outside of the film. The bases ofprotrusions 203 are parallel to smooth surface 201 a of the screen, andthe central axis of each protrusion 203 (that is, the axis runningthrough the summit of the trihedron and through the center of its base)is orthogonal to the mean plane of the film. Thus, the three surfaces ofthe trihedron are oblique with respect to the mean plane of the film.

Screen 103 differs from a conventional retroreflective screen with cubecorners in that, in screen 103, protrusions 203, instead of beingadjacent, are separated from one another by substantially smooth areas205 of surface 201 b, parallel or approximately parallel to surface 201a of the screen.

Screen 103 is intended to be illuminated by projector 105 (FIG. 1) onits surface 201 a, as schematically illustrated by arrow 207 of FIG. 2.The screen portions located opposite smooth areas 205 of surface 201 acorrespond to transparent portions of screen 103, giving way to light inboth directions with no significant deformation. The screen portionslocated opposite protrusions 203 correspond to non-transparentretroreflective portions of screen 103, capable of sending towards itssource light originating from projector 105. More particularly, when anincident light beam (not shown) reaches a retroreflective portion ofscreen 103, this beam crosses part of the screen thickness until itreaches the base of the corresponding cube corner 203, penetrates intothe cube corner, is reflected on each of the three lateral surfaces ofthe cube corner and, after reflection on the third lateral surface, setback off towards its source. In the shown example, the reflections onthe lateral surfaces of the cube corners are based on the principle oftotal internal reflection. As a variation, the lateral surfaces of thecube corners may be covered with a reflective material on the side ofsurface 201 b of the screen. The reflections on the lateral surfaces ofthe cube corners then are mirror-type reflections.

SUMMARY

An embodiment provides a retroreflective screen comprising a first filmhaving a surface comprising a plurality of microrecesses, eachmicrorecess having a bottom substantially parallel to the mean plane ofthe screen and first and second lateral walls substantially orthogonalto each other and substantially orthogonal to the bottom, the first andsecond lateral walls and the bottom of the microrecess joining at a samepoint and forming a trihedron.

According to an embodiment, in each microrecess, the first and secondlateral walls and the bottom of the microrecess are coated with areflective metallization.

According to an embodiment, the surface of the first film furthercomprises trenches with oblique or curved sides, each microrecessemerging into one of the trenches.

According to an embodiment, the trenches are V-shaped trenches formed bymachining of the surface of the first film, a plurality of microrecessesemerging into a same trench.

According to an embodiment, the first film is made of a transparentmaterial.

According to an embodiment, the first film is made of a non-transparentmaterial.

According to an embodiment, the screen further comprises a layer oftransparent glue coating the surface of the first film, and a secondfilm made of a transparent material coating the layer.

According to an embodiment, the surface coverage of the screen by themicrorecesses is lower than 50%.

According to an embodiment, the screen comprises microrecesses havingdifferent dimensions and/or orientations in different areas of thescreen.

According to an embodiment, the microrecesses are distributed accordingto a random or semi-random layout across the screen surface.

Another embodiment provides a method of manufacturing a retroreflectivescreen of the above-mentioned type, comprising manufacturing a primarymold having a surface exhibiting structures of same shape as thestructures of the surface of the first film of the screen.

According to an embodiment, the manufacturing of the primary moldcomprises a step of etching microrecesses on the side of a first surfaceof a substrate, each microrecess having a bottom substantially parallelto the mean plane of the screen and first and second lateral wallssubstantially orthogonal to each other and substantially orthogonal tothe bottom, the first and second lateral walls and the bottom of themicrorecess joining at a same point and forming a trihedron.

According to an embodiment, the manufacturing of the primary moldfurther comprises a step of forming, on the side of the surface of thesubstrate, trenches with oblique or curved sides, each microrecessemerging into a trench.

According to an embodiment, the method further comprises a step ofreplicating the patterns of said surface of the primary mold on saidsurface of the first film, by molding from the primary mold.

According to an embodiment, the replication step comprises forming asecondary mold having a shape complementary to that of the primary mold,by molding from the primary mold.

Another embodiment provides a primary mold for the manufacturing of aretroreflective screen of the above-mentioned type, having a surfaceexhibiting structures of same shape as the structures of the surface ofthe first film.

The foregoing and other features and advantages will be discussed indetail in the following non-limiting description of specific embodimentsin connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1, previously described, is a cross-section view schematicallyshowing an example of a system for displaying an image on a windshield;

FIG. 2, previously described, is a cross-section view showing in furtherdetail a screen of the system of FIG. 1;

FIGS. 3A, 4A, 5A, 6A, 7A, 8A, and 9A are perspective views schematicallyillustrating successive steps of an example of a method of manufacturingan embodiment of a screen provided with retroreflective microstructures;and

FIGS. 3B, 4B, 5B, 6B, 7B, 8B and 9B are cross-section views of thestructures of FIGS. 3A, 4A, 5A, 6A, 7A, 8A and 9A, respectively.

DETAILED DESCRIPTION

The same elements have been designated with the same reference numeralsin the different drawings and, further, the various drawings are not toscale. In the following description, when reference is made to termsqualifying absolute positions, such as terms “front”, “rear”, “top”,“bottom”, “left”, “right”, etc., or relative positions, such as terms“above”, “under”, “upper”, “lower”, etc., or to terms qualifyingdirections, such as terms “horizontal”, “vertical”, etc., it is referredto the orientation of the corresponding cross-section views, it beingunderstood that, in practice, the described devices may be orienteddifferently. Unless otherwise specified, expressions “approximately”,“about”, “substantially”, and “in the order of” mean to within 10%,preferably to within 1%, or, when angular or the like values areconcerned (for example, orientation qualifiers such as terms parallel,orthogonal, vertical, horizontal, etc.), to within 1 degree, preferablyto within 0.1°.

A limitation of screen 103 of FIG. 2 is that its retroreflectionefficiency, which is very good for angles of incidence of the projectedbeam close to the normal to the screen, for example, for angles ofincidence in the range from 0 to 20 degrees, strongly drops when theangle of incidence of the projected beam increases. As an illustration,measurements of the intensity of the retroreflected beam have beenperformed on a screen of the type described in relation with FIG. 2 fordifferent angles of the incident beam. Such measurements show that theintensity of the retroreflected beam is maximum for a zero angle ofincidence (normal incidence), that it drops to approximately 50% of itsmaximum value for a 25° angle of incidence, and that it falls to 10% ofits maximum value for a 50° angle of incidence.

This may be a problem for the application to the projection of an imageon a vehicle windshield. Indeed, in many vehicles, the windshield isstrongly inclined with respect to the vertical direction. Further, in asystem of the type described in relation with FIG. 1, projector 105assembled on the user's head may have its main projection axis inclineddownwards with respect to the horizontal direction. Thus, the angleformed between the main axis of projector 105 and screen 103 coating thewindshield may reach high values, for example, in the range from 50 to70 degrees. At such angles of incidence, the retroreflection efficiencyof screen 103 of FIG. 2 is relatively low.

Another limitation of screen 103 of FIG. 2 is its high manufacturingcomplexity, particularly due to the fact that retroreflectiveprotrusions 203 have oblique surfaces (with respect to the mean plane ofthe screen) having inclination angles which should be very accuratelycontrolled to obtain the desired retroreflective effect.

FIGS. 3A, 3B, 4A, 4B, 5A, 5B, 6A, 6B, 7A, 7B, 8A, 8B, 9A and 9Bschematically illustrate steps of a method of manufacturing anembodiment of a screen 400 provided with retroreflectivemicrostructures, compatible with a system of the type described inrelation with FIG. 1. FIGS. 3A, 4A, 5A, 6A, 7A, 8A and 9A areperspective views, and FIGS. 3B, 4B, 5B, 6B, 7B, 8B and 9B arecross-section views along plane B-B of FIGS. 3A, 4A, 5A, 6A, 7A, 8A and9A, respectively.

FIGS. 3A, 3B, 4A, 4B, 5A and 5B illustrates steps of manufacturing of aprimary mold 320 (FIGS. 5A and 5B) intended to be used on manufacturingof the actual screen 400.

FIGS. 3A and 3B illustrate a step of arranging a mask 303 on the uppersurface of a substrate 301 where primary mold 320 is desired to beformed. Substrate 301 is for example made of glass, of silicon, of athermoplastic polymer such as polymethyl methacrylate or PMMA or of anyother adapted material. The upper surface of substrate 301 is preferablyplanar. Mask 303 comprises through openings 307 exposing portions of theupper surface of substrate 301 intended to be etched in a subsequentstep. Mask 303 is made of a material capable of protecting thenon-exposed portions of substrate 301 during the subsequent etch step.As an example, mask 303 is made of metal or of resin. In top view, eachopening 307 formed in mask 303 comprises two lateral walls 308 a and 308b substantially orthogonal to each other, joining to form an angle ofapproximately 90 degrees. Each opening 307 for example has, in top view,the shape of a convex pentagon. In the shown example, each opening 307has, in top view, the shape of a right-angled isosceles triangle havinga trapezoid juxtaposed thereto. The sides of the right-angled trianglecorrespond to lateral walls 308 a and 308 b, and the base of theright-angled triangle is confounded with a base (the large base in theshown example) of the trapezoid. Openings 307 are for example allsubstantially identical and substantially oriented in the same way. Inthe shown example, only two openings 307 have been shown forsimplification. In practice, a large number of openings 307 may beprovided. As an example, openings 307 are regularly distributed acrossthe entire upper surface of substrate 301. Openings 307 are for examplearranged in an array of rows and columns.

FIGS. 4A and 4B illustrate a step of forming cavities or recesses 311extending substantially vertically in substrate 301, from its uppersurface, opposite openings 307 of mask 303. Each cavity 311 compriseslateral walls substantially orthogonal to the upper surface of substrate301, and a bottom substantially orthogonal to the upper surface of thesubstrate. In particular, each cavity comprises two lateral walls 312 aand 312 b substantially orthogonal to each other, substantiallycoinciding, in top view, with lateral walls 308 a and 308 b of openings307. Lateral walls 312 a and 312 b and bottom 312 c of each cavity 311joining at a same point S, and defining a cube corner having point S asa summit. In each cavity 311, the axis of symmetry or central axis ofthe cube corner of summit S forms, by construction, an angle ofapproximately 54.74 degrees with the upper surface of substrate 301.

As will be explained in further detail hereafter, in each cavity 311,the cube corner of summit S corresponds to a retroreflective microrecessof the future screen 400. Lateral wall 312 d of each cavity 311 oppositeto summit S (corresponding to the small base of the trapezoid of opening307 in the shown example) is preferably relatively distant from summitS, to define in cavity 311 a clearance region opposite the base of thecube corner. As an example, in top view, cavity 311 has, in thedirection of the bisectrix of the angle formed by lateral walls 312 aand 312 b, a dimension in the range from 1 to 1.5 time the cavity depth.Cavities 311 for example have a depth in the range from 20 to 500 μm andpreferably in the range from 50 to 200 μm.

Cavities 311 are for example formed by a deep reactive ion etchingmethod, generally called DRIE in the art. Such a method has theadvantage of enabling to easily form cavities having substantiallyvertical lateral surfaces down to relatively large depths, and asubstantially horizontal bottom. Any other adapted etch method mayhowever be used, for example, a laser etching or an X-ray etching.

Once the etching has been performed, mask 303 (not shown in FIGS. 4A and4B) is removed.

FIGS. 5A and 5B illustrate a step of forming trenches or recesses 314with oblique or curved sides in substrate 301, from the upper surface ofthe substrate. Trenches 314 have a depth smaller than the thickness ofsubstrate 301. Trenches 314 cross the clearance regions of cavities 311,while avoiding the cube corner regions corresponding to theretroreflective microrecesses of screen 400. Preferably, a same trench314 crosses a plurality of cavities 311. As an example, in top view,each trench 314 thoroughly crosses substrate 301 in a direction ofalignment of cavities 311 which do not cross the cube corner portions ofcavities 311. As a variation, a local trench 314 is formed at the levelof each cavity 311, that is, each trench 314 crosses a single cavity311.

Trenches 314 have a depth greater than or equal to that of cavities 311,for example, a depth in the range from 1 to 1.5 time the depth ofcavities 311. Trenches 314 preferably have a longitudinal plane ofsymmetry substantially orthogonal to the upper surface of the substrate.The depth of trenches 314, their width, and the inclination of theirsides are selected to remove all or part of the vertical walls ofcavities 311 which do not correspond to the retroreflective cube cornerregions of screen 400.

As an example, trenches 314 are V-shaped trenches. V-shaped trenches mayfor example be obtained by machining of the substrate by means of a saw,or by etching. The V-shaped trenches for example have an angularaperture in the range from 20 to 60 degrees, and preferably in the orderof 50 degrees.

As a variation, trenches 314 are trenches with curved sides, forexample, C-shaped trenches. Such trenches may for example be formed byetching.

The provision of trenches 314 enables to ease a subsequent step ofunmolding an element of screen 400 formed from mold 320. It shouldhowever be noted that trenches 314 are optional, and may in particularbe omitted if no specific unmolding difficulty arises during thissubsequent step. It should further be noted that the angle ofinclination of trenches 314 does not need to be accurately controlled,since trenches 314 are only used to ease the unmolding of the screen,but have no optical function in the final screen. The structure obtainedat the end of steps 3A, 3B, 4A, 4B, 5A, 5B corresponds to primary mold320.

FIGS. 6A and 6B illustrate a step during which the structures of theupper surface of primary mold 320 are replicated, by molding, on asurface (the upper surface in the shown example) of a film 351. As anexample, film 351 is made of a plastic material, for example, ofpolymethyl methacrylate type. In this example, film 351 is made of atransparent material. The replication of the patterns of primary mold320 of a surface of film 351 requires forming, from primary mold 320, asecondary mold (not shown) having a shape complementary to that ofprimary mold 320. The structures of the upper surface of film 351 arethen obtained, from the secondary mold, by thermoforming or by any otheradapted molding technique. For simplification, in the followingdescription, references 311, 312 a, 312 b, 312 c, 312 d, S and 314 usedto designate elements of the structures of the upper surface of primarymold 320, will be used to designate the corresponding elements of thestructures of the upper surface of film 351. The non-structured surfaceof film 351, that is, its lower surface in the shown example, ispreferably substantially planar.

FIGS. 7A and 7B illustrate a step of arranging a mask 353 on the uppersurface of film 351. Mask 353 comprises through openings 355 exposingportions of the upper surface of film 351. More particularly, openings355 are substantially arranged opposite the cube corners of summit Scorresponding to the retroreflective microrecesses of screen 400. Therest of the upper surface of film 351, and particularly the planarportions of the upper surface of film 351 which are not occupied bycavities 311 and trenches 314, as well as the portions of the uppersurface of film 351 corresponding to trenches 314 and to the clearanceregions of cavities 311, are covered with mask 353.

FIGS. 8A and 8B illustrate the result of a step of depositing, throughopenings 355 or mask 353, reflective metallizations 357 coating thelateral walls and the bottom of the cube corner microrecesses of theupper surface of film 351. Metallizations 357 are for example made ofaluminum. Metallizations 357 may be deposited by spraying through themask openings. In practice, metallizations 357 may extend on the uppersurface of film 351 slightly beyond the limit of the cube corners ofsummit S. As an example, in top view, each metallization 357 may havethe shape of a substantially circular disk having the right-angledtriangle inscribed therein corresponding, in top view, to the cubecorner recess coated with metallization 357.

Once metallizations 357 have been deposited, mask 353 is removed.

FIGS. 9A and 9B illustrate a step of bonding a transparent coating film359 to the upper surface of film 351. In this example, the surfaces offilm 359 are substantially planar. A transparent glue layer 361 filling,in particular, cavities 311 and trenches 314 of the upper surface offilm 351, forms an interface between the upper surface of film 351 andthe lower surface of film 359. Preferably, coating film 359 andtransparent glue 361 have substantially the same refraction index asfilm 351. Coating film 359 and transparent glue layer 361 enable toguarantee a good transparency of screen 400 outside of the areas coatedwith metallizations 357. The assembly thus obtained forms screen 400.

Screen 400 has retroreflective portions regularly distributed across itsentire surface, corresponding to the metallized cube corner structuresof film 351. Each retroreflective screen is surrounded with atransparent screen portion, so that the screen is partiallyretroreflective and partially transparent. Screen 400 is thus adapted toan operation of the type described in relation with FIG. 1 (with theupper surface of the screen facing projector 105). To obtain a goodtransparency of screen 400 enabling to visualize an outer scene, thesurface coverage of screen 400 by the retroreflective portions is forexample smaller than 50% and preferably smaller than 20%.

When they penetrate into screen 400 through the upper surface of film359, the incident rays are deviated by an angle which depends on theoptical index of film 359. The maximum retroreflection efficiency of thecube corner structures of screen 400 is in principle obtained when therays projected on the structures are parallel to the central axis of thecube corners of summit S, that is, when the rays propagating inside ofthe screen are inclined by approximately 54.74 degrees with respect tothe mean plane of the screen. Such an inclination of the rays within thescreen can generally not be obtained in practice, since this inclinationis greater than the limiting refraction angle of the upper diopter ofthe screen. As an example, for a film 359 having an optical index in theorder of 1.5, the limiting refraction angle is approximately 42 degrees.It should further be noted that the higher the angle of incidence of thelight rays on screen 400, the more significant the losses by reflectionon the upper surface of screen 400. According to the selected materials,one may easily find, by measurement and/or simulation, the angle ofincidence of the light rays for which the retroreflection efficiency ofthe screen is maximum. As an example, measurements have shown that, whenfilms 359 and 351 and glue layer 361 have a refraction index in theorder of 1.5, the maximum retroreflection efficiency of screen 400 isobtained for an angle of incidence (outside of the screen) in the orderof 60 degrees. More generally, the tests which have been performed showthat the provided structure provides a good retroreflection efficiencyfor angles of incidence in the range from 30 to 80 degrees, andpreferably in the range from 50 to 70 degrees. Thus, screen 400 is welladapted to the application to the projection of an image on a vehiclewindshield.

Another advantage of screen 400 is that it is relatively simple to form,due to the fact that the cube corner microrecesses forming theretroreflective portions of the screen comprise no oblique surfaces withrespect to the mean plane of the screen. The surfaces of the cube cornermicrorecesses of screen 400 are substantially orthogonal or parallel tothe mean plane of the screen. Thus, the cube corner microrecesses may beobtained by means of a simple etching with vertical sides from the uppersurface of substrate 301.

Specific embodiments have been described. Various alterations,modifications, and improvements will occur to those skilled in the art.In particular, the described embodiments are not limited to theabove-mentioned example where the cube corner microrecesses forming theretroreflective portions of screen 400 are substantially identical andoriented in the same way. In practice, according to the needs of theapplication, cube corner microrecesses may have different dimensionsand/or different orientations (in top view) in different areas of thescreen.

Further, the cube corner microrecesses forming the retroreflectiveportions of screen 400 are not necessarily aligned in rows and incolumns, but may have a random or semi-random distribution on the screensurface, particularly to avoid possible diffraction phenomena capable ofoccurring under certain angles of incidence when the microrecesses areregularly arranged.

Further, the described embodiments are not limited to the example of amethod of manufacturing reflective metallizations 357 described inrelation with FIGS. 7A, 7B, 8A, 8B. As a variation, the step of formingmask 353 described in relation with FIGS. 7A and 7B may be omitted.Instead, a conformal metal layer coating the entire upper surface of thestructure of FIGS. 6A and 6B may be formed. The deposited metal can thenbe removed from the upper planar regions of the structure (correspondingto the transparent areas of the screen), for example, by chem.-mech.polishing. During this step, only the portions of the metal layercoating the walls of cavities 311 and, possibly, of trenches 314, arekept, forming reflective metallizations 357.

It should further be noted that in an application of the type describedin relation with FIG. 1, light source 105 is generally not placedexactly in the axis of the user's viewing angle. Thus, screen 400 shouldpreferably be capable of diffusing the retroreflected light in adiffusion cone encompassing the user's pupil, so that the user can seethe image displayed by the projector. In practice, the inventors haveobserved that diffraction effects on the edges of the microrecessesand/or unavoidable surface imperfections of the screen may be sufficientto obtain the required diffusion effect. To amplify and/or control sucha diffusion, the roughness of the sides and of the bottom of cavity 311of primary mold 320 may for example be varied.

Further, the described embodiments are not limited to the application tothe projection of an image on a transparent surface. In particular, thedescribed embodiments may have applications in various fields usingretroreflective surfaces, not necessarily transparent, for example, forsignaling purposes. In certain cases, it may indeed be desirable to havea surface with a good retroreflection efficiency for high angles ofincidence. As an example, such a surface may be useful for groundsignaling applications in the field of roads for motor vehicles. In thecase where the transparency is not desired, it will preferably bedesired to maximize the screen surface coverage by the cube cornerretroreflective portions. Further, the material of film 351 may be nontransparent. Coating film 359 and intermediate glue layer 361 shouldhowever be transparent to allow the incident light to reach cube cornermetallizations 357 and then to come out of the screen after beingreflected on the metallization surfaces. As a variation, coating film359 and intermediate glue layer 361 may be omitted. Further, althoughthe screen transparency is not required, reflective metallizations 357are not necessarily located on the cube corner portions of substrate351, but may form a continuous layer formed by conformal deposition andcoating the entire upper surface of substrate 351.

It should be noted that in the present description, term film has beenused to designate elements 351 and 359 of screen 400. This term shouldhowever be understood in a wide sense, and particularly includeselements similar to films such as sheets, plates, etc.

Further, as a variation, instead of identically replicating thestructures of the surface of primary mold 320 (FIGS. 5A, 5B) on theupper surface of film 351 (FIGS. 6A and 6B), structures having shapescomplementary to those of primary mold 320 may be formed on the uppersurface of film 351. In other words, rather than forming a secondarymold from primary mold 320 and thus molding the upper surface of film351 from the secondary mold, the upper surface of transparent film 351may be directly molded from primary mold 320. In this case, cube cornermicroprotrusions are obtained on the upper surface side of film 351.Film 351 then forms a retroreflective screen intended, as in the exampleof FIG. 2, to be illuminated on its lower surface. When an incidentlight beam (not shown) reaches a retroreflective portion of the screen,the beam crosses part of the screen thickness to reach the base of thecorresponding cube corner protrusion, penetrates into the cube corner,is reflected on each of the third lateral surfaces of the cube corner,and, after reflection on the third lateral surface, sets back offtowards its source. The reflections on the lateral surfaces of the cubecorners may be based on the principle of total internal reflection. As avariation, the lateral surfaces of the cube corners may be covered witha reflective material on the upper surface side of the screen, thereflections on the lateral surfaces of the cube corners then beingmirror-type reflections.

Thus, in a variation, a retroreflective screen comprising a first filmmade of a transparent material having a surface comprising a pluralityof microprotrusions, each microprotrusion having a first surfacesubstantially parallel to the mean plane of the screen, and second andthird surfaces substantially orthogonal to each other and substantiallyorthogonal to the first surface, the first, second, and third surfacesof the microprotrusions joining at a same point and forming a trihedron.

The first, second, and third surfaces of each microprotrusion may becoated with a reflective metallization.

Said surface of the first film may further comprises strips with obliqueor curved sides, each microprotrusion being placed against one of thestrips.

As an example, a plurality of microprotrusions are placed against a samestrip.

The screen surface coverage by the microprotrusions is for example lowerthan 50%.

The screen may comprise microprotrusions having different dimensionsand/or orientations in different screen areas.

The microprotrusions (311) are for example distributed according to arandom or semi-random layout across the screen surface.

Such alterations, modifications, and improvements are intended to bepart of this disclosure, and are intended to be within the spirit andthe scope of the present invention. Accordingly, the foregoingdescription is by way of example only and is not intended to belimiting. The present invention is limited only as defined in thefollowing claims and the equivalents thereto.

What is claimed is:
 1. A retroreflective screen comprising a first filmmade of a transparent material, having a surface comprising a pluralityof microprotrusions, each microprotrusion having a first reflectivesurface substantially parallel to a mean plane of the screen, and secondand third reflective surfaces substantially orthogonal to each other andsubstantially orthogonal to the first surface, the first, second, andthird surfaces of the microprotrusions joining at a same point andforming a trihedron.
 2. The retroreflective screen of claim 1, whereinthe first, second, and third surfaces of each microprotrusion are coatedwith a reflective metallization.
 3. The retroreflective screen of claim1, wherein said surface of the first film further comprises strips withoblique or curved sides, each microprotrusion being placed against oneof the strips.
 4. The retroreflective screen of claim 3, wherein aplurality of microprotrusions are placed against a same strip.
 5. Theretroreflective screen of claim 1, wherein the screen surface coverageby the microprotrusions is lower than 50%.
 6. The retroreflective screenof claim 1, wherein the surface of the first film comprisesmicroprotrusions having different dimensions and/or orientations indifferent screen areas.
 7. The retroreflective screen of claim 1,wherein the microprotrusions of the first film are distributed accordingto a random or semi-random layout across the screen surface.
 8. A methodof manufacturing the retroreflective screen of claim 1, comprisingmanufacturing a mold having a surface exhibiting structures having shapecomplementary to that of the structures of said surface of the firstfilm of the screen.
 9. The method of claim 8, wherein the manufacturingof the mold comprises a step of etching microrecesses on the side of afirst surface of a substrate, each microrecess having a bottomsubstantially parallel to the mean plane of the screen and first andsecond lateral portions substantially orthogonal to each other andsubstantially orthogonal to the bottom, the first and second lateralwalls and the bottom of the microrecess joining at a same point andforming a trihedron.
 10. The method of claim 9, wherein themanufacturing of the mold further comprises a step of forming, on theside of said surface of the substrate, trenches with oblique or curvedsides, each microrecess emerging into a trench.
 11. The method of claim8, further comprising a step of realizing, on said surface of the firstfilm, patterns being complementary to the patterns of said surface ofthe mold, by molding from the mold.